With fossil fuels becoming depleted and global warming occurring due to combustion thereof, development of new energies that can take the place of fossil fuels is becoming an urgent issue. Solar energy is a clean, environment-friendly energy source having adequate potential to support sustained development of future generations. As a method for converting solar energy to electricity, silicon-based semiconductor solar cells are being developed. However, the silicon used in such solar cells is required to be extremely high in purity and because a vast amount of energy is spent in a refining process and the process is complex, a high production cost is required.
Due to having comparatively high conversion efficiencies and being low in cost in comparison to conventional solar cells, dye-sensitized solar cells are becoming noted both in academia and business. In particular, with a dye-sensitized solar cell reported in 1991 by Graetzel et. al., a photoelectric conversion efficiency of 10 to 11% is achieved. With the dye-sensitized solar cell, a dye is adsorbed onto nanotitania particle surfaces to enable absorption of light in the visible range, and the role of the dye is particularly important due to exhibiting a light capturing action. As such dyes, cis-bis(isothiocyanato)-bis(2,2′-bipyridine-4,4′-carboxylate) ruthenium (II), which is denoted as N3, cis-bis(isothiocyanato)-bis(2,2′-bipyridine-4,4′-carboxylate) ruthenium (II) bis(tetra-n-butylammonium), which is denoted as N719, and cis-bis(isothiocyanato)-(2,2′-bipyridine-4,4′-carboxylate)-(2,2′-bipyridine-4,4′-dinonyl) ruthenium (II), which is denoted as Z907, have been developed until now. Although the dyes N3 and N719 exhibit high photoelectric conversion efficiencies, these have a problem in durability at a comparatively high temperature. This is due to the dye molecules desorbing from the titanium dioxide particle surfaces induced by the attack of water molecules, etc. On the other hand, Z907 is a type of amphiphilic ruthenium dye and is a dye of excellent stability. However, Z907 had a demerit of being low in photoelectron conversion efficiency due to the low absorption spectrum intensity in a visible to ultraviolet range in comparison to N3 and N719.
Although recently published ruthenium dyes include a ruthenium complex having a pyridylquinoline derivative as a ligand (Patent Document 1), a ruthenium complex having a terpyridine derivative as a ligand (Patent Documents 2 to 4), etc., none of these can improve both the photoelectric conversion efficiency and the durability.
Furthermore, very recently, Graetzel et. a1. reported a photosensitizer dye with a specific structure having a 2,2′-bipyridine ligand (Patent Documents 5 and 6 and Non-Patent Document 3). However, because the absorption spectrum intensity in the visible-ultraviolet range still cannot be said to be adequate and the photoelectric conversion efficiency is low in a correlation to a redox potential, further improvement of the photoelectric conversion efficiency is desired.
Also, a wide range of photosensitizer dyes with a specific structure having a 2,2′-bipyridine ligand are reported in Patent Document 7. However, it is considered that further improvement is required in regard to stability of the actually cited dyes.
Development of a ruthenium complex or other transition metal complex as a new photosensitizer dye exhibiting strong absorption over a wider spectral range and being excellent in stability is thus desired.    Patent Document 1; Japanese Patent Laid-Open No. 2003-272721    Patent Document 2: Japanese Patent Laid-Open No. 2003-212851    Patent Document 3: Japanese Patent Laid-open No. 2005-47857    Patent Document 4: Japanese Patent Laid-Open No. 2005-120042    Patent Document 5: European Patent Application Publication No. 1622178    Patent Document 6: International Patent Publication No. WO 2006/010290    Patent Document 7: Japanese Patent Laid-Open No. 2001-271534    Non-Patent Document 1; J. Phys. Chem. B 2003, 107, 14336-144337    Non-Patent Document 2: Nature Material, p 402, Vol. 2, 2003    Non-Patent Document 3: J. Am. Chem. Soc. 2005, 127, 808-809